Understanding the impact of the central atom on the ionic liquid behavior: Phosphonium vs ammonium cations

Abstract

Density relaxation and particle motion characteristics in a non-ionic deep eutectic solvent (acetamide + urea): Time-resolved fluorescence measurements and all-atom molecular dynamics simulations J. Chem. Phys. 142, 034505 (2015) The influence of the cation's central atom in the behavior of pairs of ammonium-and phosphoniumbased ionic liquids was investigated through the measurement of densities, viscosities, melting temperatures, activity coefficients at infinite dilution, refractive indices, and toxicity against Vibrio fischeri. All the properties investigated are affected by the cation's central atom nature, with ammonium-based ionic liquids presenting higher densities, viscosities, melting temperatures, and enthalpies. Activity coefficients at infinite dilution show the ammonium-based ionic liquids to present slightly higher infinite dilution activity coefficients for non-polar solvents, becoming slightly lower for polar solvents, suggesting that the ammonium-based ionic liquids present somewhat higher polarities. In good agreement these compounds present lower toxicities than the phosphonium congeners. To explain this behavior quantum chemical gas phase DFT calculations were performed on isolated ion pairs at the BP-TZVP level of theory. Electronic density results were used to derive electrostatic potentials of the identified minimum conformers. Electrostatic potential-derived CHelpG and Natural Population Analysis charges show the P atom of the tetraalkylphosphonium-based ionic liquids cation to be more positively charged than the N atom in the tetraalkylammonium-based analogous IL cation, and a noticeable charge delocalization occurring in the tetraalkylammonium cation, when compared with the respective phosphonium congener. It is argued that this charge delocalization is responsible for the enhanced polarity observed on the ammonium based ionic liquids explaining the changes in the thermophysical properties observed. © 2014 AIP Publishing LLC.

Hydrogen-bond acidity of ionic liquids: an extended scale

“…This charge distribution also has a significant impact on other properties, such as in the phase diagrams of binary IL-water systems, densities and viscosities, as previously shown. 60,61 On the other hand, the C-H bonds of the alkyl groups in tetraalkylphosphonium ILs are hardly polarised compared to tetraalkylammonium, which is clearly shown by the low α-value of the former IL. This behaviour can be attributed to the different charge distribution/contribution of the ammonium-and phosphonium-based IL cations as previously demonstrated by Carvalho et al 60,61 The higher acidity afforded by ammonium-based ILs also correlates well with their lower ability to form two-aqueous phase systems reflected by a higher ability to hydrogenbond with water.…”

“…60,61 On the other hand, the C-H bonds of the alkyl groups in tetraalkylphosphonium ILs are hardly polarised compared to tetraalkylammonium, which is clearly shown by the low α-value of the former IL. This behaviour can be attributed to the different charge distribution/contribution of the ammonium-and phosphonium-based IL cations as previously demonstrated by Carvalho et al 60,61 The higher acidity afforded by ammonium-based ILs also correlates well with their lower ability to form two-aqueous phase systems reflected by a higher ability to hydrogenbond with water. 62 Two sets of imidazolium-based ILs were also studied: one with 1-methyl-3-alkylimidazolium-based asymmetric ILs and the other consisting of 1,3-dialkylimidazoliumbased symmetric ILs.…”

Development of predictive QSAR models for Vibrio fischeri toxicity of ionic liquids and their true external and experimental validation tests

“…16,17,20,[22][23][24][25][26][27][28][29][30][31][32][33][34][35][36] Table S1 of the ESI. † While the other descriptors were obtained without the need of any geometry optimization process, the QTMS parameters were derived from the ab initio based optimized geometry at the HF/6-31G(d) level of theory, and were limited to only cations.…”

The aquatic impact of ionic liquids on freshwater organisms

“…Usually, the increase in the alkyl chain length leads to a greater negative impact (Garcia et al, 2005;Latala et al, 2009a). Regarding the cation core, some studies report that aromatic cations (imidazolium and pyridinium) normally are more toxic than non-aromatic ILs, such as pyrrolidinium, piperidinium (Ventura et al, 2013), phosphonium and ammonium (Carvalho et al, 2014). Although sometimes neglected, the toxicity of anions such as trifluoromethanesulfonate, dicyanamide (Steudte et al, 2012), tetrafluoroborate (Pinto et al, 2012) and bis(trifluoromethanesulfonyl)imide (Costa et al, 2015) has been demonstrated.…”